Electroviscous fluids are fluids which change their viscosity when a voltage is applied to them. This property is used, for example, in mechanically moving control parts such as automobile clutches, brakes and engine mountings. Examples of such fluids which have been studied in the prior art are compositions wherein starch, silica gel and polyacrylates are dispersed in a non-conducting medium. The specific gravity of the dispersoid was however high compared to that of the dispersion medium, and it therefore tended to settle out to form a sediment. Other compositions have been reported wherein sedimentation is avoided by changing the type of solid component or increasing its blending proportion, but these are better described as pastes rather than fluids. In U.S. Pat. Nos. 4,033,892, 4,129,513 and Japanese patent Kokoku (Examined Japanese Patent Publication) No. 26, 151/'88, a composition is disclosed consisting mainly of an acrylic polymer as a dispersoid; in British Patent No. 1,076,754 and Japanese Patent Kokai (Unexamined Japanese Patent Publication) Nos. 259,752/'86 and 44,998/'86, a composition is disclosed consisting of a silica or silica gel dispersoid; in Japanese Patent Kokai No. 95,397/'87, a composition is disclosed consisting of an aluminium silicate dispersoid; in British Patent No. 3, 047,507 and Japanese Patent Kokai No. 32,197/'83, a composition is disclosed consisting mainly of a barium titanate dispersoid; and in Japanese Patent Kokai No. 33,459/'88, a composition is disclosed consisting of a polyacrylamide dispersion. Of these compositions, those wherein the dispersoid is an acrylic polymer have an excellent electrical field response, and are also known to perform very well in mechanically moving control parts. However, these dispersoids have a high true specific gravity of 1.3-1.6. They therefore suffered from the disadvantage that they settled out with time, and it was consequently difficult to obtain compositions which were stable over long periods. One method proposed to overcome this defect was to use a substance of high specific gravity, for example trifluorovinyl chloride polymer, polychlorinated biphenyl or orthodichlorobenzene, as the dispersion medium.
These dispersion media were however harmful to persons or the environment, and some of them had relatively high volatility. They were therefore not necessarily suitable.
Further, in Japanese Patent Kokai 275699/'89, a method is proposed wherein a dye is dispersed in a mixture of a silicone oil and a fluorinated oil of similar specific gravity to make a suspension.
In this case, to improve the storage stability of the dispersoid, very strict quality control is necessary to ensure that the specific gravity of the dispersion medium is the same as that of the dispersoid, the dye. This presented a problem in manufacture.
Further, various dispersion stabilizers have been proposed to control the above sedimentation (Japanese Patent Kokai 44,998/'86 and 95,397/'87), but the addition of these substances also caused a decline in the electrical insulating properties of the composition. The inventors of the present invention, after detailed studies, found that by dispersing a powder of an addition polymer derived mainly from an acrylate or methacrylate monomer in a 2 phase fluid consisting of a mixture of a silicone oil and a synthetic fluorinated oil, sedimentation of the dispersoid could easily be prevented, and a long-life electroviscous fluid composition could be obtained.